How-to make large graph more readable?

Question

One day, I found myself curious about different topics in mathematics. So, I stumbled uppon a website (considerably old website) where author has tabularized mathematics into subjects and associated topics.

Based on the information taken from the website, I decided to make a Graph to see how do different subjects and topics relate.

I managed to load the data:

web = Import["http://www.math-prof.com/", "Data"];

and retrieve the data:

rawData = web[[2]][[2 ;; All]];

From the above raw set of data, I have retrieved all subjects:

subjects = If[(Length @ #) > 1, List @ First @ #, List @ #] &  [First @  rawData [[#]]] & /@ Range @ ((Length @ rawData) - 1); (*-1 because I wasn't interested in Physics bit*)

The topics were a bit more troublesome so some manual work has been carried out to clean-up the dataset

From the above set of files, I carried out an Import:

topics = Import[ "t" <> ToString @ # <> ".txt", "List"] & /@ Range @ 12;

Finally, creating a graph:

Graph[Flatten @ Table[Rule[subjects[[i]] // First , #] & /@ topics[[i]], {i, Length @ subjects}], VertexLabels -> "Name", EdgeShapeFunction -> "Line"]

Here is the output:

I was wondering how I could make this Graph more readable? As you see, the Text is practically unreadable.